In web offset printing presses, a substrate such as a web of paper is sequentially driven through a series of print cylinders, each using ink of a different color, which cooperate to imprint a multicolor image on the web. To provide an accurate and clear multicolor image, the rotational and lateral position of each print cylinder must be precisely aligned, i.e., proper color registration of the respective colors must be maintained.
Color registration control systems for printing presses are known in the art. An example of a closed-loop color registration control system is the commercially available RGS V From QTI of Sussex, Wis. The RGS V system provides a closed-loop color registration control system employing an optical line scanner which cooperates with paper movement to provide, in effect, a two-dimensional raster scan of a predetermined portion of the web on which registration marks are imprinted by the respective print cylinders.
In general, a color registration control system interacts with a printing press to keep a plurality colors in registration, i.e., lining up the colors on top of each other while the colors are being printed. Most printing presses use three basic subtractive primary colors (yellow, magenta, cyan) and black to create a printed image. Special print colors can also be utilized. There are several reasons why the print may not be in register. For instance, the printing plates may not be mounted or setup on the plate cylinder correctly. Dynamics such as tension, stretch, ink coverage, and web weave can in turn introduce a color register error between different printing units.
Typically, color registration control systems includes a scanning unit for acquiring images of the substrate being printed upon, a processing unit for searching for color register marks and image processing the acquired images, a conventional shaft encoder and a suitable motor controller. The registration control system generates control signals to an adjustment mechanism in accordance with the relative positions of the registration marks. The system then provides appropriate signals to the electric motors to precisely control lateral and rotational position of the various plate cylinders.
However, the processing unit and the scanning unit are generally housed in different devices in different locations on or near the printing press. For example, the scanning unit is often mounted above the web and the processing unit is often located in a different location. The devices must therefore be interfaced by running video cables between them. It is normally difficult to transmit an image from the scanning unit to the processing unit without distortion, with the distance between the devices further contributing to the degradation of the high quality image processing desired. Further, difficulties exist in transmitting such large amounts of data.
Tracking is another registration control system concern resulting from the remote relative placement of the scanning unit and the processing unit. It can be difficult to setup and install the scanning unit properly on a printing press. Since the alignment of the scanning unit is important and the scanning unit depth-of-field is shallow, readjustment may be required when the scanning units are changed.
Synchronization can also be a challenge. Existing registration control systems typically attempt to synchronize a free-running camera with standard video output (e.g. RS-170) to a strobe and the web-position encoder. Precise synchronization can be difficult because it requires synchronization between a plurality of devices in the control system including proper lighting and the scanning unit.
Specifically, the synchronization becomes difficult because some area scanning units are not re-triggerable. They simply continuously read out frame data. The problem in using these scanning units for any image recognition is that the scanning unit is typically running at a constant 30 Hz that is totally asynchronous to the speed of the printing press. There is no guarantee that the scanning unit is in the right part of the printing cycle when the mark pattern is directly under the lens of the scanning unit. A typical compensation procedure is to keep the ambient lighting detected by the scanning unit relatively dark, and then activate a strobe light based upon the encoder pulse count at a desired time.
Such scanning units work in such a way that they generally have a light sensitive image area and a storage area. The light sensitive area is accumulating charge (exposing/integrating) while the storage area is being read out. In other words, a current frame is always being exposed while a previous frame is being read. There is a lag time in between frames when the charge of the current frame is being transferred from the imaging area into the storage area. If the strobe activates during this part of the cycle, the image contains total darkness if the strobe duration is entirely within a time between frames, or it contains some amount of partial darkness if the strobe duration partially overlaps the time between frames and partially overlaps the frame time. Neither of these is desirable because it is difficult to identify if the dark image is caused by the synchronization problem or if it is an indication that the light source is too dark and hence requires adjustment. Synchronization is based upon an interaction between the printing press speed and the frequency at which the color register marks are showing up under the lens of the scanning unit with the frequency of the scanner itself. At certain press speeds, a high percentage of images would be partially dark just due to the interaction of these two frequencies.
To overcome the discrepancy between the two frequencies, re-triggerable scanning units can be considered. This involves interrupting the frame/field that is being read, and restarting the sequence based upon a pulse re-triggering rate. However, the re-triggering rate is often measured in large multiples of microseconds or even milliseconds, and often re-triggering is coupled with clearing the sensor charge in preparation for a fresh exposure. These steps take time and result in the printed register marks moving a long distance in this period of time at high press speed. A solution is to provide an anticipator circuit that re-triggers the scanning unit at a number of encoder pulses before the actual encoder pulse at which the picture is to be taken. When the actual encoder count occurs, a strobe trigger is activated. However, the number of pulses required in the anticipation is dependent on the press speed, and this complicates the system design, its implementation and flexibility.
In typical color registration control systems, each printing unit of a printing press prints at least one registration mark of a predetermined size and shape on a predetermined portion of the web, typically along its edge. When in proper registry, the registration marks from the individual print units will be in predetermined relative disposition or pattern on the web. Some registration control systems adopt a normalized nominal reference coordinate system with a Y axis parallel to the direction of web movement and an X axis parallel to the scan lines. Deviation of marks from such relative dispositions is indicative of a registration error, i.e. misregistration. For example, deviation from an expected X value is indicative of lateral misregistration, and deviation from the expected Y value is indicative of circumferential misregistration.
Color registration marks can have various configurations such as a right angle diamond (i.e. a square rotated by 45 degrees) and various sizes such as 0.04″ or 0.06″ diamonds. Symmetrically shaped register marks facilitate a determination of a predetermined point associated with the mark, e.g., the center points of the mark.
A lighting source is typically employed to illuminate the web in order for the scanning unit to acquire a useable image of the web. A plurality of high-intensity light sources such as tungsten-halogen bulbs can be used to illuminate the web, and especially the registration marks printed on the web. Many existing color registration systems utilize two light sources or bulbs in at attempt to achieve high-intensity, uniform illumination. In a two bulb system, the light source illumination characteristics have to be matched, and moved away from the lens to provide uniformity of the lighting. The cost of maintaining two light sources is also high.
Once the web has been illuminated, the scanning units are then focused on the illuminated portion of the web. The scanning units generally include optical line or area scanners cooperating with suitable circuitry for controllably driving the scanning unit, such as suitable transfer pulse synchronization logic, conventional CCD driver circuitry, conventional buffer circuitry, and a video analog-to-digital converter.
Color registration control systems are typically designed to provide a closed-loop control that automatically converges to target settings and maintains color registration throughout the entire print run. Some color registration control systems may either need to be told where to find the register marks, have limited searching capability, and/or require many plate revolutions to find the register marks. Accordingly, make-ready time can be lengthy which results in waste of material and time.